Thanks. What's the widest good lens for the NEX-7 that won't get the issues of colour casting?

As others have said, Sigma 19mm f/2.8 will perform better than most SLR 20mm f/2.8 lenses and is much less money than the ones that might perform comparably. The SLR lenses you are paying a lot for extra frame coverage. I think the Nikkor 20mm f/3.5 I had was about the same at f/8 as the Sigma was at f/2.8, maybe it was a little better than that, but not much.

Not any particular width, more lens design. The issue with color casts on the NEX 7 sensor is that the light hits the sensor too steep of an angle so some of the light reflects away from the sensor. This has to do with size of rear lens elements and how far they are from the sensor.

The Sigma 19mm has what is known as an image space telecentric design (or close to it). This means the light hits at less steep of an angle. All SLR wides will be similar since they sit a long ways from the sensor. Some rangefinder lenses use lenses with simpler optic designs but small rear sensors that cause issues in the corners. So almost all range finder and short flange compact wide angles will be problematic, including native lenses. The CZ 24mm f/1.8 is pretty large because it has a design that works well with the Sony E mount.